Many reversible, variable displacement, axial piston pumps have a pivotal swash plate to both change the direction of delivery of hydraulic fluid from the pump and to adjust the displacement of the pump. Some of these variable displacement pump designs use a pair of opposed operating hydraulic servo actuators to control the position of the swash plate. Usually those variable displacement pumps have some sort of servo valve mechanism for controlling the flow of pressurized pilot fluid to and from the servo actuators wherein the servo valve mechanism is returned to a position so that the swash plate is maintained at a position corresponding to an input force or pressure applied to the servo valve mechanism.
The recent trend in pump controls is to use either pilot or electro-hydraulic control of the servo valve mechanism. The servo valve mechanism of those pumps generally includes a valve spool slidably disposed in a moveable sleeve which is either connected to the swash plate directly or through a linkage. One pilot operated system for controlling the servo mechanism has a pair of servo pistons for rotating a lever which in turn moves the valve spool in the appropriate direction depending upon which one of the servo pistons is actuated. Moving the valve spool directs pressurized fluid to one of the servo actuators to pivot the swash plate for changing the displacement and direction of delivery of the pump. The pivotal movement of the swash plate in turn moves the sleeve to a blocking position to stop the fluid flow to the servo actuator, thereby causing the swash plate to stop and be held at a desired position. One of the disadvantages of such servo valve mechanisms is that very tight tolerances must be held on the valve spool and sleeve thereby increasing the manufacturing cost of the pump. Moreover, the forces required to move the spool through such lever arrangement necessitates the use of relatively large servo pistons and relatively stiff springs to center the servo pistons in their neutral position. Such servo valve mechanisms require numerous adjustments to the servo mechanism after all the pump components are assembled, thereby increasing the complexity of adjusting the swash plate to the centered or zero displacement position. Furthermore, the fluid commonly used for powering the hydraulic actuators is low pressure pilot fluid. This results in a disadvantage since the servo actuators must be relatively large to provide sufficient force to overcome the swivel torques exerted by the pump pistons.
The servo valve mechanisms of some reversible variable displacement pumps are combined in cartridge form with one of the hydraulic actuators. Such servo valve mechanisms are complicated and difficult to adjust to the centered position of the swash plate.
The present invention is directed to overcoming one or more of the disadvantages or problems as set forth above.